Ionic liquids are “salt-like” materials that are liquid at relatively low temperatures (less than 400° C.). Welton, T. Chem. Rev. 1999, 99, 2071-2083; Binnemans, K. Chem. Rev. 2005, 105, 4148-4204. Today most scientists consider an ionic liquid a salt that is a liquid below 100° C. Many of the common ionic liquids have melting points at room temperature or below. The first ionic liquids prepared were corrosive materials that had limited utility. In the early 1990s, Wilkes reported a less corrosive air-stable ionic material; this result has provided the impetus for much of the research and development focused on ionic liquids. Wilkes, J. S.; Zaworotko. M. J. Chem. Commun. 1992, 965-967.
Ionic liquids are typically composed of a mono-cationic organic compound, such as a compound based on the structure of a imidazolium, pyridinium, pyrrolidinium, phosphonium, ammonium or sulfonium, and an inorganic or organic anion, such as a alkyl sulfate, tosylate, methansulfonate, hexafluorophosphate, tetrafluoroborate, halide, or carboxylic acid. For example, the prototypical ionic liquid of 1-ethyl-3-methylimidazolium ethyl sulfate has a melting point of less than 20° C.; whereas, sodium chloride has a melting point of 801° C. The strong ionic interaction between these mono-cations and mono-anions results in low vapor pressure, non-flammable materials with high thermal, mechanical, and electrochemical stability. More recently, a dicationic organic compound with two mono-anions has been reported. Han, X.; Armstrong, D. W. Org. Lett. 2005, 7(19), 4205-4208.
Ionic liquids have found uses in a wide range of applications including, but not limited to, lubricants, MADLI-tof matrices, protein crystallization matrices, solvents for heterogeneous catalysis, solvents for homogeneous catalysis, solvent for organic synthesis, solvents for desulfurization, liquid crystals, thermal fluids, fuel cells, sensors, metal finishers, materials for gas separations, distillation fluids, extraction mediums, and membrane technology.
Viscoelastic materials are semi-solid materials which exhibit solid and liquid like properties depending on temperature, as well as upon being subjected to stress and strain over time. These materials act like elastic solids under some conditions and viscous liquids under others. In other words, viscoelastic materials are materials that show hysteresis in their stress-strain curves. Many plastics and crosslinked polymers are viscoelastic materials. Examples that highlight the diversity of known viscoelastic materials are skin, most of the soft tissue in the body, and the memory foam which is found in matrices.